At least nine cases of apparent preschool manic-depressive illness have been previously reported in the literature. In each of these children, a strong family history of affective illness was noted. In this report, the case histories of six preschool children ages 3 to 5 years with bipolar illness are summarized. These six were obtained from a sample of 36 consecutively hospitalized preschool children. Thus 17% of these hospitalized preschool children had bipolar illness. All had irritable mood, strong family history of affective illness, and previous presentation with symptoms of attention deficit hyperactivity disorder. They were diagnosed following a thorough clinical interview. Five children were treated with lithium; all five improved. Preschool mania exists as an identifiable entity and may respond to classic pharmacologic treatments.